1. Field
This disclosure relates generally to integrated circuits, and more specifically, to forming gate dielectrics for transistors of an integrated circuit.
2. Related Art
Gate dielectrics are a very significant factor in the performance of a transistor and thus the performance of an integrated circuit made up of such transistors. The historic preferred gate dielectric has been a silicon oxide grown at very high temperature. This is often called a thermally grown oxide. Reducing the thickness of a gate dielectric is a known way of enhancing coupling between gate and channel and thus improving speed but adversely effects leakage and reduces breakdown voltage. Thus much work has been done in developing high K dielectrics for use as a gate dielectric so that gate to channel coupling is enhanced but enough thickness is retained to have low leakage and reasonable breakdown voltage. The most promising of the high K materials have been various metal oxide. There have been many difficulties in obtaining the desired result for metal oxides. One problem is growing an excessively large interfacial oxide layer at the interface with the substrate, which is most commonly silicon. This interfacial oxide layer is very difficult to prevent completely, but it is preferably as thin as possible. In order to minimize the interfacial oxide thickness, the oxygen concentration applied in the formation of the metal oxide is kept as low as possible to form the metal oxide. A problem arising from keeping the oxygen concentration relatively low is that the resulting metal oxide layer has oxygen vacancies, locations where oxygen atoms should be present but are not present. The oxygen vacancies, when excessive, cause a reliability problem. With usage, the threshold voltage shifts. The direction of the shift depends on the particular metal oxide used, but it is a reliability problem either way. If the magnitude of the threshold voltage decreases, the transistors may not turn off completely and cause excessively leakage and may even result in a logic operational failure. If the magnitude of the threshold voltage increases, the speed of operation can decrease excessively and either reduce the speed of operation below the required speed or even cause an operational failure.
Nitrogen has been used to fill the oxygen vacancies and reduce Hf dangling bonds to prevent the attendant reliability issues. A benefit of using nitrogen is that there is also an increase in the dielectric constant. A problem with nitrogen is that nitrogen atoms at the interface with the substrate can reduce the mobility in the channel. Thus, it is desirable for nitrogen to be present in most of the thickness but not at the interface with the substrate. This has been difficult to achieve, especially while continuing to minimize interfacial oxide growth. Ammonia NH3 as a source of nitrogen does well in filling the vacancies but reaches the substrate very easily so the resulting nitrogen concentration is undesirably high at the interface with the substrate. In addition, hydrogen release during an ammonia anneal degrades gate oxide quality.
Accordingly, there is a need for a method for obtaining nitrogen in a metal oxide gate dielectric that avoids or reduces one or more of the problems described above.